In a photocopier, printer, fax machine and image forming apparatus using electrophotographic process such as a multi-functional peripheral including the functions of these devices, the latent image corresponding to a document is formed on a photoreceptor drum and toner is applied to this latent image, whereby the image is developed by toner. The developed toner image is transferred onto a recording material. After that, the toner image of the recording material is fixed and the sheet is ejected.
When a color image is formed, four latent images Y, M, C and K corresponding to document colors are formed on four photoreceptor drums, and four developed toner images are primarily transferred onto an endless intermediate transfer belt. After that, these images are transferred secondarily onto a recording material. The toner image transferred onto the recording material is fixed and the sheet is ejected.
One of the fixing apparatuses for fixing above-mentioned toner image is a heat roller fixing type fixing apparatus, wherein the recording material with the toner image transferred thereon is sandwiched in between and conveyed using the nip portion made up of a fixing roller incorporating a heating device such as a halogen heater, and a pressure roller for pressing the fixing roller, whereby the recording material is heated and pressed. This type of fixing apparatus has come into widespread use because of simple structure.
Incidentally, in such a fixing apparatus, a sufficient heat must be supplied to the toner and recording material in order to increase the speed. This requires the nip width to be increased. To increase the nip width, it would be necessary to increase the load of the pressure roller for applying pressure to the fixing roller, the thickness of the elastic layer made up of silicone rubber and others in the pressure roller or the diameters of two rollers.
However, if the load of the pressure roller or the thickness of the elastic layer has been increased, uneven nip width may result in the axial direction. This may cause uneven fixing and wrinkles of the recording material to be produced. Further, an increase of the roller diameter causes the fixing apparatus to be increased in size and the warming-up time to be prolonged.
One of the apparatuses disclosed to solve these problems is a fixing apparatus including:
a rotating fixing roller which contains an elastic layer formed of a silicone rubber and incorporates a heating device such as a halogen heater at the center;
an endless fixing belt driven and rotated by a fixing roller; and
a pressure pad arranged on the inner peripheral surface side of the fixing belt, wherein the fixing belt is pressed toward the fixing roller by a pressure pad (e.g., Japanese Unexamined Patent Application Publication No. 2006-91182).
According to this fixing apparatus, the fixing belt pressed against the fixing roller by the pressure pad is subjected to elastic deformation, and a wider nip portion is formed between the fixing roller and fixing belt. This arrangement is capable of meeting high speed requirements, without the fixing apparatus being increased in size. Further, the heat capacity of the fixing belt is small. This feature reduces the warming-up time and saves energy.
In the fixing apparatus disclosed in the Japanese Unexamined Patent Application Publication No. 2006-91182, when the inner peripheral surface of a rotating fixing belt is made to slide along the fixed pressure pad, the sliding resistance will be increased. This may result in a reduction in the fixing image quality due to misalignment of a fixing belt, an increase in the power consumption of a motor for driving the fixing belt due to an increase in the drive torque of the fixing belt, damages of a speed reduction gear, abrasion on the inner peripheral surface of the fixing belt, or a similar trouble. Thus, to minimize the sliding resistance between the fixing belt and pressure pad, a sliding sheet made of low-friction material is arranged between the fixing belt and pressure pad. Further, a lubricant supply member for supplying a lubricant is provided, wherein this lubricant supply member is arranged in close contact with the inner peripheral surface of the fixing belt.
In addition, the inner peripheral surface of the fixing belt is formed into a rough surface. In this case, an increase in surface roughness on the inner peripheral surface of the fixing belt will reduce the contact area with the sliding sheet. This will lead to a reduction in the sliding resistance with the sliding sheet, and a friction noise is increased by contact between the fixing belt and sliding sheet. By contrast, if the surface roughness on the inner peripheral surface of the fixing belt is reduced, the friction noise can be reduced, but the effect of reducing the sliding resistance is also reduced.
In the fixing apparatus disclosed in the Japanese Unexamined Patent Application Publication No. 2006-91182, for the surface roughness value for the inner peripheral surface of the fixing belt, the percentage of the maximum peak height to the maximum valley depth is defined by a predetermined conditional expression. Further, the maximum height obtained by adding the maximum valley depth to the maximum peak height is also defined by a predetermined conditional expression, whereby both the sliding resistance and rubbing noise can be reduced at the same time.
In the meantime, the present inventors have made concentrated study efforts to study the relationship between the surface roughness of the inner peripheral surface of the fixing belt and reduction in the fixing belt drive torque, have found out the conditional expression of the surface roughness defined in the Japanese Unexamined Patent Application Publication No. 2006-91182 is not sufficient to reduce the drive torque. To be more specific, even if the measurement has successfully met the conditional expression of the Japanese Unexamined Patent Application Publication No. 2006-91182, the drive torque is sometime low and sometimes high, depending on the difference in the roughened structure on the surface. A positive means cannot be provided by defining the inner peripheral surface of fixing belt in above-mentioned manner, according to the findings by the present inventors.
To be more specific, above-mentioned conditional expression merely defines the height of the minute peak on the surface of the fixing belt, i.e., the height of the protruding section, and the depth of the valley, i.e., the depth of the recessed section. No consideration is given at all to the expansion of the protruding section and recessed section in the lateral direction. Thus, in the recessed section of minute depth, lubricant leaks from the recessed section by the surface tension. This makes it difficult to ensure positive holding of the lubricant. Thus, even when the conditional expression of the surface roughness on the inner peripheral surface of a fixing belt, positive holding of the lubricant is difficult when there are a great number of recessed sections of minute width. Drive torque cannot be reduced, according to the test conducted by the present inventors.
In view of the problems described above, it is an object of the present invention to provide a fixing belt wherein the surface roughness of the inner peripheral surface of the fixing belt is designed with consideration given not only to the height of the protruding section and the depth of the recessed section but also to the expansion of the protruding section and recessed section in the lateral direction so that the contact resistance, hence, the sliding resistance can be reduced, and stable holding of the lubricant can be ensured, whereby the drive torque of the fixing belt can be reduced; a fixing apparatus equipped with this fixing belt; and an image forming apparatus equipped with this fixing apparatus.